Vapor degreasing and solvent cleaning with fluorocarbon based solvents have found widespread use in industry for the degreasing and otherwise cleaning of solid surfaces, especially intricate parts and difficult to remove soils.
In its simplest form, vapor degreasing or solvent cleaning consists Of exposing a room temperature object to be cleaned to the vapors of a boiling solvent. Vapors condensing on the object provide clean distilled solvent to wash away grease or other contamination. Final evaporation of solvent from the object leaves behind no residue as would be the case where the object is simply washed in liquid solvent.
For difficult to remove soils where elevated temperature is necessary to improve the cleaning action of the solvent, or for large volume assembly line operations where the cleaning of metal parts and assemblies must be done efficiently and quickly, the conventional operation of a vapor degreaser consists of immersing the part to be cleaned in a sump of boiling solvent which removes the bulk of the soil, thereafter immersing the part in a sump containing freshly distilled solvent near room temperature, and finally exposing the part to solvent vapors over the boiling sump which condense on the cleaned part. In addition, the part can also be sprayed with distilled solvent before final rinsing.
Vapor degreasers suitable in the above-described operations are well known in the art. For example, Sherliker et al. in U.S. Pat. No. 3,085,918 disclose such suitable vapor degreasers comprising a boiling sump, a clean sump, a water separator, and other ancillary equipment.
Cold cleaning is another application where a number of solvents are used. In most cold cleaning applications, the soiled part is either immersed in the fluid or wiped with rags or similar objects soaked in solvents and allowed to air dry.
Fluorocarbon solvents, such as trichlorotrifluoroethane, have attained widespread use in recent years as effective, nontoxiC, and nonflammable agents useful in degreasing applications and other solvent cleaning applications. Trichlorotrifluoroethane has been found to have satisfactory solvent power for greases, oils, waxes and the like. One isomer of trichlorotrifluoroethane is 1,1,2-trichloro-1,2,2-trifluoroethane (known in the art as CFC-113) It has therefore found widespread use for cleaning electric motors, compressors, heavy metal parts, delicate precision metal parts, printed circuit boards, gyroscopes, guidance systems, aerospace and missile hardware, aluminum parts and the like.
The art has looked towards azeotropic compositions including the desired fluorocarbon components such as trichlorotrifluoroethane which include components which contribute additionally desired characteristics, such as polar functionality, increased solvency power, and stabilizers. Azeotropic compositions are desired because they do not fractionate upon boiling. This behavior is desirable because in the previously described vapor degreasing equipment with which these solvents are employed, redistilled material is generated for final rinse-cleaning. Thus, the vapor degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e., is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the cleaning and safety of processing. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not an azeotrope or azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency towards soils, less inertness towards metal, plastic or elastomer components, and increased flammability and toxicity.
U.S. Pat. No. 3,455,835 discloses an azeotropic composition of about 54 to about 64 percent by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and from about 36 to about 46 percent by weight of trans-1,2-dichloroethylene. This teaching does not suggest the present azeotrope-like composition because as is known in the art, it is difficult to predict if another component will form a new azeotrope with a known azeotrope.
Another example is taught in U.S. Pat. No. 4,767,561. The disclosed composition comprises from about 4 to about 72 percent by weight of 1,1,2-trichloroethane-1,2,2-trifluoroethane; from about 23 to about 29 percent by weight of trans-1,2-dichloroethylene; and from about 5 to about 7 percent by weight of methanol.
While azeotropic or azeotrope-like compositions are useful as cleaning solvents, the azeotrope-like compositions should be stabilized against possible changes during storage and use. One potential change is due to chlorofluorocarbons such as trichlorotrifluoroethane hydrolyzing to form HCl. When metallic materials are present such as occurs in many cleaning applications, the problem is worsened because the metal acts as a catalyst and causes the hydrolysis of trichlorotrifluoroethane to increase expotentially. Metallic materials such as Al-2024, copper, cold rolled steel, galvanized steel, and zinc are commonly used in cleaning apparatus. Another potential change is due to ultraviolet light decomposing chlorofluorocarbons such as trichlorotrifluorocarbons.
One example of a stabilized azeotrope-like composition is taught in U.S. Pat. No. 4,804,493. The disclosed composition comprises from about 54 to about 64 percent by weight of 1,1,2-trichloro-1,2,2-trifluoroethane and from about 36 to about 46 percent by weight of trans-1,2-dichloroethylene and effective stabilizing amounts of 4-methoxyphenol: 1,2-butylene oxide; and nitromethane.
Another example of a stabilized azeotrope-like composition is taught in U.S. Pat. No. 4,803,009. The disclosed composition comprises from about 64 to about 72 weight percent of 1,1,2-trichloro-1,2,2-trifluoroethane; about 23 to about 29 weight percent of trans-1,2-dichloroethylene; and about 5 to about 7 weight percent of methanol and effective stabilizing amounts of 4-methoxyphenol: 1,2-butylene oxide; nitromethane; and diisopropylamine. The patent cites U.S. Pat. No. 3,960,746 which teaches that the combination of 1,1,2-trichloro-1,2,2-trifluoroethane and lower alcohols attacks reactive metals such as zinc and aluminum and thus, the composition of U.S. Pat. No. 4,803,009 includes nitromethane to retard this attack.
Other references which teach azeotrope-like compositions having the combination of 1,1,2-trichloro-1,2,2-trifluoroethane and alcohol stabilized by nitromethane to retard attack of metallic materials include U.S. Pat. No. 3,789,006 (1,1,2-trichloro-1,2,2-trifluoroethane; isopropanol; and nitromethane); U.S. Pat. No. 3,903,009 (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; and nitromethane); U.S. Pat. No. 3,960,746 (1,1,2-trichloro-1,2,2-trifluoroethane; methanol; and nitromethane); commonly assigned U.S. Pat. No. 4,062,794 (1,1,2-trichloro-1,2,2-trifluoroethane; methanol; ethanol; isopropanol; and nitromethane); commonly assigned U.S. Pat. No. 4,052,328 (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; isopropanol; and nitromethane); U.K. Pat. No. 2,066,840B (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; nitromethane; and acetone) Japanese Pat. Nos. 81-34,799 and 81-34,79B (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; nitromethane; and 3-methylpentane or 2,2-dimethylbutane or 2,3-dimethylbutane); Japanese Pat. No. 81-109,298 (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; nitromethane; and n-hexane); commonly assigned U.S. Pat. No. 4,606,841 (1,1,2-trichloro-1,2,2-trifluoroethane; ethanol; nitromethane; acetone; and hexane); and commonly assigned U.S. Pat. No. 4,683,075 (1,1,2-trichloro-1,2,2-trifluoroethane; methanol; nitromethane; acetone; and methyl acetate).